The present invention relates to methods of acidizing subterranean formations or well bores, and more specifically, to acidizing systems involving acid-generating fluids that comprise acid-generating compounds that generate acids, and associated methods. More particularly, the present invention relates to controlling the generation rate of the acid from the acid-generating compound.
Acidizing and fracturing treatments using aqueous acidic solutions commonly are carried out in subterranean formations (including those that contain hydrocarbons as well as those that do not) penetrated by well bores to accomplish a number of purposes, one of which is to increase the permeability of the formation. The resultant increase in formation permeability normally results in an increase in the recovery of hydrocarbons from the formation.
Acidizing techniques can be carried out as “matrix acidizing” procedures or as “acid fracturing” procedures. Generally, in acidizing treatments, aqueous acidic solutions are introduced into the subterranean formation under pressure so that the acidic solution flows into the pore spaces of the formation to remove near-well formation damage and other damaging substances. The acidic solution reacts with acid-soluble materials contained in the formation which results in an increase in the size of the pore spaces and an increase in the permeability of the formation. This procedure commonly enhances production by increasing the effective well radius. When performed at pressures above the pressure required to fracture the formation, the procedure is often referred to as acid fracturing. Fracture-acidizing involves the formation of one or more fractures in the formation and the introduction of an aqueous acidizing fluid into the fractures to etch the fractures' faces whereby flow channels are formed when the fractures close. The aqueous acidizing fluid also enlarges the pore spaces in the fracture faces and in the formation. In fracture-acidizing treatments, one or more fractures are produced in the formation and the acidic solution is introduced into the fracture to etch flow channels in the fracture face. The acid also enlarges the pore spaces in the fracture face and in the formation. The use of the term “acidizing” herein refers to both types of acidizing treatments, and more specifically, refers to the general process of introducing an acid down hole to perform a desired function, e.g., to acidize a portion of a subterranean formation or any damage contained therein.
Although acidizing a portion of a subterranean formation can be very beneficial in terms of permeability, conventional acidizing systems have significant drawbacks. One major problem associated with conventional acidizing treatment systems is that deeper penetration into the formation is not usually achievable because, inter alia, the acid may be spent before it can deeply penetrate into the subterranean formation. The rate at which acidizing fluids react with reactive materials in the subterranean formation is a function of various factors including, but not limited to, acid strength, acid concentration, temperature, fluid velocity, mass transfer, and the type of reactive material encountered. Whatever the rate of reaction of the acidic solution, the solution can be introduced into the formation only a certain distance before it becomes spent. For instance, conventional acidizing fluids, such as those that contain mineral acids, organic acids of suitable strength, hydrochloric acid or a mixture of hydrofluoric and hydrochloric acids, have high acid strength and quickly react with the formation itself, fines and damage nearest the well bore, and do not penetrate the formation to a desirable degree before becoming spent. To achieve optimal results, it is desirable to maintain the acidic solution in a reactive condition for as long a period of time as possible to maximize the degree of penetration so that the permeability enhancement produced by the acidic solution may be increased.
Another problem associated with acidizing subterranean formations is the corrosion caused by the acidic solution to any metal goods (such as tubular goods) in the well bore and the other equipment used to carry out the treatment. For instance, conventional acidizing fluids, such as those that contain organic acids, hydrochloric acid or a mixture of hydrofluoric and hydrochloric acids, have a tendency to corrode tubing, casing and down hole equipment, such as gravel pack screens and down hole pumps, especially at elevated temperatures. The expense of repairing or replacing corrosion damaged equipment is extremely high. The corrosion problem is exacerbated by the elevated temperatures encountered in deeper formations. The increased corrosion rate of the ferrous and other metals comprising the tubular goods and other equipment results in quantities of the acidic solution being neutralized before it ever enters the subterranean formation, which can compound the deeper penetration problem discussed above. The partial neutralization of the acid results in the production of quantities of metal ions which are highly undesirable in the subterranean formation.
Another problem associated with conventional acidizing systems is that they can pose handling and/or safety concerns due to the reactivity of the acid. For instance, during a conventional acidizing operation, corrosive fumes may be released from the acid as it is injected down the well bore. The fumes can cause an irritation hazard to nearby personnel, and a corrosive hazard to surface equipment used to carry out the operation.
To combat these problems, acid-generating compounds have been contemplated for use in acidizing fluids. In this way, a live acid is not used. Rather, the acid is delayedly generated downhole for use in these and other various applications. Although such approaches may be beneficial, they may still have some drawbacks in that the delay achieved by using the acid-generating compound may not be optimized. Moreover, there is a need to accurately control the hydrolysis time of the acid-generating compound to be able to get the acid to the portion of the subterranean formation in which it is needed. Some of the acid-generating compounds contemplated, such as diethylene glycol diformate, may generate an acid too fast for placement. Others, such as ethyl lactate, may generate an acid too slow. The particular reaction rate depends on the particular acid-generating compound, the temperature of application, and the concentration of the acid-generating compound. Of the preceding criteria, only the concentration of the acid-generating compound is easily controlled, and its effect is very minor compared to the other two criteria. What is needed is an additional means to control the generation of the acid so that it the generated acid can be used as desired in a subterranean formation.
Additionally, when using/making completion fluids, the choice of what brine to use is dictated by the overall weight of the fluid. However, optimally, the choice of brine also should be guided by the effect it may have on an acid-generating compound in the brine in order to achieve the desired reaction rate for the acid-generating compound so that the desired acidizing effect is achieved.